1. Field of the Invention
The invention is generally related to pile driving and more particularly to the design of pile driving transition pieces.
2. General Background
Pile driving hammers, particularly hydraulic hammers designed to drive pipe piles in the offshore environment, are of two types. One type has an external sleeve that enables the hammer to be cantilevered from the top of the pile that is being driven, sometimes known as a free riding hammer. The other type has a constant diameter that is equal to the diameter of the pile that is being driven, sometimes known as a slimline hammer. The free riding and slimline hammers are represented in FIGS. 1 and 3, respectively. The slimline hammer cannot cantilever from the pile top and must be supported by guides. The external guide and stabbing bell of the free riding hammer cannot clear support guides, so the pile must cantilever the free riding hammer above any obstructions.
A free riding hammer can be used to drive a battered pile. A couple at the level of the pile driving head and the bottom of the external sleeve of the hammer develops the necessary cantilever moment. The arrows in FIG. 1 represent the couple. Frequently, in pile driving operations a pile is driven to partial penetration with a smaller hammer and driven to final penetration with a larger hammer. Often, the external sleeve of the smaller hammer will not fit over the pile, which fits the larger hammer. To solve this problem a pile driving transition piece is stabbed into the pile top and the smaller hammer is stabbed over the smaller top end of the transition piece, as shown in FIG. 2. The hammer cantilevers from the transition piece, developing the couple C1 represented by the arrows acting on the top end in FIG. 2. The transition piece cantilevers from the pile top, developing the couple C2 represented by the arrows acting on the transition piece stabbing point 23. A greater pile batter will develop a greater couple acting on the stabbing point. Making the stabbing point longer reduces the magnitude of the couple.
Historically, transition piece stabbing points have had a fatigue problem, which gets worse as the stabbing points are made longer. A fatigue crack 24 forms a few inches below the driving shoulder and runs circumferentially around the stabbing point, causing the stabbing point to break off and fall into the interior of the pile. If the pile is battered enough, the transition piece and hammer will fall off the top of the pile once the stabbing point breaks off. The typical location for the fatigue crack 24 is indicated in FIG. 2.
A slimline hammer, as seen in FIG. 3, can only fit one diameter of pile. To use a slimline hammer on a pile with a diameter greater than the hammer diameter presents a support problem because the support guides must be sized for the larger diameter pile. Then the guides are too large to support the smaller diameter hammer.
The invention addresses the above needs of preventing fatigue cracks and allowing a slimline hammer to be used on more than one pile diameter. What is provided is a shock avoiding pile driving transition piece. The transition piece separates the function of delivering the driving energy of the hammer to the pile top from the function of cantilevering the hammer and transition piece from the top of the pile. A shock isolation unit provides the only axial connection between the transition cantilever element and the driving element. The connection is made with an elastomeric material. The material is resilient, so that when a blow is struck on top of the driving element, the driving energy passes through the driving element to the pile top without a significant portion of the energy being diverted into the cantilever element.